Thermal insulating material and method of producing same

ABSTRACT

The invention relates to a thermochemically stable oxidic thermal insulating material presenting phase stability, which can be used advantageously as a thermal insulating layer on parts subjected to high thermal stress, such as turbine blades or such like. The thermal insulating material can be processed by plasma spraying and consists preferably of a magnetoplumbite phase whose preferred composition is MMeAl 11 O 19 , where M is La or Nd and where Me is chosen from among the alkaline earth metals, transitional metals and rare earths, preferably from magnesium, zinc, cobalt, manganese, iron, nickel and chromium.

[0001] The invention relates to a thermal insulating material, which isparticularly suited for high temperature applications far above 1000° C.and can be employed in gas turbines, aeroplane engines, power stationturbines and other highly, thermally loaded parts, for example invehicle construction and energy technology.

[0002] The invention further relates to a method for producing andprocessing such a thermal insulating material.

[0003] The known thermal insulating materials specifically employed forhigh temperature applications in heat-power machines and in industrialplants are oxide cover layers applied to a metal substrate, for exampleon a highly alloyed nickel base material in a turbine blade. Theclassical thermal insulating layer consists of tetragonal or stabilisedZrO₂ as a cover layer, which is usually applied to an additionalintermediate layer in the form of a low melting point or soft couplinglayer (HVS). The coupling layer is composed substantially of aluminumand yttrium, frequently also with amounts of platinum and palladium (upto 10 wt.-%), apart from further components (nickel, chromium, cobalt),to make it more oxidation resistant. The ceramic cover layer is mostoften applied by atmospheric plasma spraying (APS). Newer developmentsconcern ZrO₂ layers vapour-deposited with electron beams (electron beamsphysical vapour deposition, EB-PVD-ZrO₂-layers). The requirements on theceramic ZrO₂ cover layer and the coupling layer have increasedcontinuously in recent years. Their stability under alternatingtemperatures, their protective effect against oxidation as well as theirlong-term stability and adhesion at higher temperatures of the exhaustgas for increased efficiency have been optimised.

[0004] As a disadvantage of the known thermal insulating layers on thebasis of ZrO₂ it has been found that layers applied by plasma sprayingor CDV and EB-PVD layers of stabilised ZrO₂ are not sufficientlyresistant above 1100° C. The ZrO₂ layers age rapidly at temperaturesabove 1100° C.

[0005] This aging process leads to a partial densification of the layerand parallel to that the elasticity modulus of the layer increases. Thedensity increase diminishes the original uniform fine porosity of thelayer and the thermal conductivity increases. The increase in elasticitymodulus of the ceramic layer means that the thermal shock resistancedecreases and the “tolerance” or the capability of compensating forthermal expansion with highly different thermal expansion co-efficientsbetween the ceramic layer and the metallic substrate decreases. Bothprocesses, the density increase and the increase in elasticity moduluslead to a peeling of the ZrO₂ cover layer during the temperature cyclesin a turbine.

[0006] In addition to deterioration the pure mechanical properties ofthe cover layer, the three dimensional sintering of the ZrO₂ layer leadsto the formation of a dense ceramic with other properties than that ofthe porous layer. Since ZrO₂ is a very good conductor of ions, the everpresent oxidative degradation in the entire ceramic-metal composite isnot altered by densification of the ceramic. The coupling layer oxidisesin this process and a layer of oxidation products with other propertiesforms between the original coupling layer and the ceramic cover layer.The original ceramic layer thus in the end breaks up due to the alteredmechanical properties of the layer system. The corrosion of the couplinglayer continues despite the sometimes very dense ceramic surface.

[0007] The object of the present invention is therefore to provide animproved thermal insulating material which is better suited forhigh-temperature applications and is particularly suited for coatingturbine blades and similar high temperature components.

[0008] Furthermore, a suitable method for producing and processing suchthermal insulating materials is to be provided.

[0009] According to the invention, this object is solved by a thermalinsulating material composed of at least one first component with atleast one first phase containing stoichiometrically 1 to 80 mol-% ofM₂O₃, 0 to 80 mol-% MeO and a remainder of Al₂O₃ with incidentalimpurities, wherein M is selected from the elements lanthanum andneodymium or mixtures thereof and wherein Me is selected from alkalineearth metals, transition metals and the rare earths or mixtures thereof,preferably selected from magnesium, zinc, cobalt, manganese, iron,nickel, chromium, europium, samarium or mixtures thereof.

[0010] An effective thermal insulation is made possible with theinsulating material of the invention, also at temperatures of 1300° C.and up to over 1500° C., whereby at the same time sintering processesand the resultant ageing and grain enlargement compared to ZrO₂ aregreatly slowed down or retarded.

[0011] In a preferred embodiment of the invention, the first componentcontains 1 to 80 mol-% M₂O₃ and 0.5 to 80 mol-% MeO with a remainder ofAl₂O₃.

[0012] It has been shown to be of advantage when the first componentcomprises 1 to 50 mol-% M₂O₃ and 1 to 50 mol-% MeO with a remainder ofAl₂O₃.

[0013] It is further preferred when the first component comprises 1 to20 mol-% M₂O₃ and 2 to 30 mol-% MeO with a remainder of Al₂O₃.

[0014] Furthermore, it has been shown to be of advantage when the firstcomponent comprises 2 to 20 mol-% M₂O₃ and 5 to 25 mol-% MeO with aremainder of Al₂O₃.

[0015] Particularly preferred is the first component comprising 5 to 10mol-% M₂O₃, about 10 to 20 mol-% MeO with a remainder of Al₂O₃.

[0016] Particularly advantageous properties result when the firstcomponent comprises about 5 to 9 mol-% M₂O₃, 12 to 17 mol-% MeO with aremainder of Al₂O₃, whereby a composition with 7.1 mol-% M₂O₃, about14.3 mol-% MeO and a remainder of Al₂O₃ represents an optimalcomposition.

[0017] The first phase preferably forms a hexa-aluminate phase ofmagnetoplumbite structure of the composition MMeAl₁₁O₁₉, which whenusing lanthanum as M and magnesium as Me is known as magnesium aluminumlanthanum oxide with the formula MgAl₁₁LaO₁₉.

[0018] This material consists mainly of aluminum oxide in whichmonolayers of lanthanum oxide and aluminum oxide are disposed at regularspacings. This insertion of La₂O₃ leads to the formation of a layeredstructure with a characteristic plate-like structure of the crystals.This magnetoplumbite phase only forms in a narrowly restrictedcomposition region. The typical composition LaAl₁₁O₁₈ due to itsstructure has very many cationic (about 8% Al) and anionic (about 5% O)vacancies in the lattice, which allow the diffusion of atoms through thestructure. The homogeneity region of the phase is extended toLaMgAl₁₁O₁₉by doping with bivalent cations having a small ionic radius(typically Mg⁺⁺, Mn⁺⁺, Co⁺⁺, Zn⁺⁺, etc.). In this ideal compositionLaMgAl₁₁O₁₉ the compound has nearly no more possibility of altering itscomposition.

[0019] With a further increase in the doping with MgO and La₂O₃ (or MeOand M₂O₃) further defects form in the structure and a multiphase regionforms including LaMgAl₁₁O₁₉, MgAl₂O₄, LaAlO₃ and MgO.

[0020] In the optimal composition according to the invention, theaddition of MeO leads to a decrease in the lattice vacancies. This meansthat the material with the composition LaMgAl₁₁O₁₉ (MMeAl₁₁O₁₉) hasabsolutely no more crystal defects in the structure or formulated inanother way, all of the vacancies in the structure are occupied by Me(Mg) and an additional O atom. This complete occupancy of all latticesites in the structure leads to the desired high thermo-chemicalstability and phase stability in the temperature region above 1100° C.

[0021] A further important advantage of the thermal insulating materialof the invention is that the material is substantially inert withrespect to alkali compounds (Na₂O, NaCl, K₂O, KCl) of the combustion gasor the surrounding atmosphere.

[0022] Previous thermal insulating materials based on ZrO₂ form lowmelting point phases with the hydroxides or carbonates of Na₂O and K₂Oor with the NaCl contained in the atmosphere in winter or near the sea,which lead to a strong densification of the sprayed layer attemperatures of 1000° C. In contrast, such attacks on the thermalinsulating material of the present invention lead more to an increasedplate growth, which subsequently makes densification, i.e. the sinteringof the cover layer substantially more difficult.

[0023] A further advantage of the thermal insulating material of thepresent invention is a favourable thermal expansion coefficient, whichlies between 9.5 and 10.7×10⁻⁶ [K⁻¹] in a temperature range between roomtemperature and 1200° C. and thus in a range favourable for coatinghighly heat resistant steels, which have an expansion coefficient ofabout 10 to 12×10⁻⁶ [K⁻¹].

[0024] With the thermal insulating material of the present invention,the application of a thin, very effective thermal insulating layer ispossible on a body, for example made of chromium nickel steels, whichhave an exceptionally high temperature resistance and long-termstability and by which a peeling of the thermal insulating layer fromthe base material is effectively avoided even after numerous thermalcycles.

[0025] The thermal insulating material of the present invention ispreferably applied by thermal spraying, in particular by plasma sprayingas a thermal insulating layer on a body to be coated.

[0026] To achieve a preferred crystallisation of the aluminate duringthe plasma spraying and to increase adhesion and thermal shockresistance, the first component can additionally be doped by a secondcomponent, which preferably is substantially insoluble in thehexa-aluminate phase and preferably is added to the first component inan amount of about 0.001 to 20 wt.-%, in particular about 0.1 to 10wt.-%, whereby the range of 0.1 to about 3 wt.-% is particularlypreferred.

[0027] The second component can comprise at least one of the compoundsZrO₂ in monoclinic, tetragonal or cubic form, La₂Zr₂O₇, MgZrO₃, Nd₂O₃,HfO₂, Y₂O₃, Yb₂O₃, Eu₂O₃, La₂Hf₂O₇, MgHfO₃, oxides or salts of thealkali metals sodium, potassium, lithium or mixtures or alloys of thesecompounds.

[0028] If ZrO₂ is added in tetragonal or cubic form, then preferably itis doped with MgO, CaO or Y₂O₃.

[0029] Concerning the salts of the alkali metals sodium, potassium andlithium, which can also be added as a doping of the first component,these can be carbonates, chlorides, nitrates, acetates, formates,citrates, sulphates, hydrogen carbonates or mixed salts of these metals.

[0030] The thermal insulating material of the present invention ispreferably first produced in powder form and then subsequently appliedas a thermal insulating material to a component, for example by plasmaspraying, or is processed to produced massive components using powdertechnology methods or is further processed to a ceramic foam.

[0031] According to a first alternative, the powder-like thermalinsulating material is produced by adding an insoluble oxide, ahydroxide or an oxygen hydrate of Al₂O₃ as the starting material to anaqueous or alcoholic medium, in particular methanol, ethanol orisopropanol, the remaining portions of the first component being solublesalts, preferably carbonates, hydrogen carbonates or acetates. Thestarting material is dissolved in the medium, the formed suspension isdried, preferably after a grinding and dispersion step, preferably spraydried, and the resulting powder is subsequently subjected to anannealing treatment.

[0032] A relatively uniform distribution and a good mixing of thevarious additives is achieved in this wet chemistry process, by which aninsoluble carrier powder is coated. The subsequent annealing treatmentis preferably carried out at temperatures of 500 to 1800° C. in thepresence of air for a duration between about 0.5 and 20 hours. Theannealing process, for example in a rotary oven, produces asingle-phase, oxidic agglomerate with an average diameter of betweenabout 1 and 200 μm and with a specific surface area between 0.1 and 40m²/g.

[0033] According to a second alternative for the powder production, thecompounds of the first component are mixed in powder form as oxides orsalts in a mixer, preferably a drum or tumbling grinder, wherepreferably grinding bodies of Al₂O₃ or stabilised ZrO₂ are employed. Thepowder is subsequently granulated and subjected to an annealingtreatment.

[0034] This so-called “mixed oxide method” is the simplest variation forproduction, however, it is somewhat more difficult to obtain ahomogeneous mixture. At first the produced powder still has severalphases even after the mixing process.

[0035] The multiphase oxide mixture is preferably treated with bindersand granulated before the annealing treatment is carried out, which isalso preferably performed in the presence of air, preferably for aduration between 0.5 and 20 hours in a temperature range of betweenabout 300° C. and 1800° C.

[0036] A homogeneous oxidised powder is formed by the annealing process,where the granulates have an average diameter of between about 1 and 200μm and a specific surface area between 0.1 and 40 m²/g.

[0037] Conversely, if the mixing is performed as mentioned above in aliquid medium or one works with a suspension having a high content onsolids, then a drying is initially carried out, preferably by spraydrying, before the subsequent annealing treatment.

[0038] A third alternative for producing the powder-like thermalinsulating material is producing the powder by a sol-gel process withsubsequent drying and annealing.

[0039] A particularly good chemical homogeneity and a complete phasetransition during the annealing is achieved when using a sol-gelprocess. The powder produced by the sol-gel process is particularly finegrained and is well suited for subsequent processing by powdertechnology methods or by plasma spraying.

[0040] In the sol-gel process, alcoholates are preferably produced fromthe starting materials in the desired mass ratios, subsequently solidcomponents are precipitated out of the solution, preferably by theaddition of water or by pH adjustment. The solid components aresubsequently separated from the excess solution and dried and thenannealed at temperatures between about 500° C. and 1200° C.

[0041] In a variation of this process, organic binding agents areadditionally added after the precipitation of the solid components andthen the subsequent separation of excess solution takes place, beforethe drying, preferably spray drying, and finally the annealing treatmentfollows. The annealing is preferably performed at temperatures between500° C. and 1200° C.

[0042] In both variations, alcoholate compounds of the form(—OC_(n)H_(2n−1)) are used, whereby —OC_(n)H_(2n+1) means methoxy,ethoxy, isopropoxy, propoxy, butoxy or isobutoxy alcoholates with 1≦n≦5.

[0043] Alternatively, water soluble salts of M (lanthanum or neodymium)or Me (in particular magnesium), preferably as an acetate, citrate,carbonate, hydrogen carbonate, formate, hydroxide or nitrate, can beadded to a solution of aluminum alcoholate and subsequentlyprecipitated.

[0044] If the first component is doped with the second component, thistakes place according to a further embodiment of the invention in theliquid state, in which the second component is added in soluble formbefore the drying or precipitation takes place (as long as the sol-gelprocess is being used).

[0045] In contrast, in the dry method (mixed oxide method) the compoundsof the second component are added as a powder and annealed together withthe other compounds and are brought to chemical reaction in the solidphase.

[0046] As mentioned above, the thermal insulating material of thepresent invention can be applied either in powder form by plasmaspraying onto the part to be coated or can be subsequently processedusing powder technology methods, for example by axial cold pressing,isostatic cold pressing or slip casting and subsequent sintering,preferably under a slightly reducing atmosphere at temperatures of atleast about 1500° C. or by extruding or casting foils with thecorresponding subsequent heat treatment to produce larger articles.

[0047] According to another variation of the invention, the powder canalso be produced in a ceramic foam, namely by filling a polymer foamwith slip whereafter the solvent is preferably evaporated attemperatures between 200° C. and 400° C. or by adding a suspension ofthe powder to a low viscosity polymer, which is then foamed with afoaming agent, and finally in both variations carrying out an annealingtreatment, preferably at first in a range between 900° C. and 1100° C.and finally at about 1400° C. to 1700° C.

[0048] It will be understood that the features of the invention are notonly applicable in the given combinations but may also be used in othercombinations or taken alone without departing from the scope of theinvention.

[0049] The invention will now be discussed in more detail in conjunctionwith the drawings.

[0050]FIG. 1 shows an illustration of the crystallographic unit cell ofthe magnetoplumbite phase;

[0051]FIG. 2 shows a schematic illustration of the occupation of amirror plane in the magnetoplumbite phase;

[0052]FIG. 3 shows the phase diagram of the system La₂O₃/Al₂O₃/MgO (inmol-%);

[0053]FIG. 4 shows a REM image of a lanthanum magnetoplumbite specimenafter tempering at 1570° C. for 10 hours;

[0054]FIG. 5 shows the results of an EDX analysis, which was carried outwith an electron scanning microscope and shows results of a corrosiontest with Cr₂O₃;

[0055]FIG. 6 shows the results of an EDX analysis, which was carried outwith an electron scanning microscope and shows results of a corrosiontest with NiO;

[0056]FIG. 7 shows the results of an X-ray powder diffraction scan on apowder produced via the wet route, from which the magnetoplumbite phaseis easily verified on the basis of the JCPDS cards for the LaA₁₁O₁₈phase and the LaMgAl₁₁O₁₉ phase according to Table 1 and Table 2;

[0057]FIG. 8 shows a flow diagram for powder production according to themixed oxide method;

[0058]FIG. 9 shows a flow diagram for powder production according to thewet chemistry method;

[0059]FIG. 10 shows a flow diagram for powder production fromalcoholates via a sol-gel process;

[0060]FIG. 11 shows a flow diagram of production variations, by whichthe first component is additionally doped with a second component toproduce a powder highly capable of crystallisation; and

[0061] FIGS. 12-15 show REM images of a specimen of a powder made in thewet method generated by sintering.

STRUCTURE OF THE THERMAL INSULATING MATERIAL ACCORDING TO THE INVENTION

[0062] The thermal insulating material of the present invention consistsof an oxidic cover layer, which in contrast to the zirconium oxidesinteres not in three dimensions, but preferably in two dimensions. Thematerial consists mainly of aluminum oxide, where monolayers oflanthanum oxide or neodymium oxide and aluminum oxide are disposed inits crystal lattice at regular spacings (see FIG. 1 and FIG. 2). Theinsertion of La₂O₃ leads to the formation of a layered structure with avery characteristic, plate-like structure of the crystals which isclearly seen in FIG. 4. The hexa-aluminates form a magnetoplumbite phasein a very restricted region of the composition. A typical composition isLaA₁₁O₁₈. However, this composition due to its structure has manycationic (Al) and anionic (O) vacancies in the lattice, which allow adiffusion of atoms through the structure. By doping with bivalentcations having a small ionic radius (typically MgO, MnO, CoO, ZnO,generally referred to as MeO), the homogeneity of the phase is extendedup to MMeAL₁₁O₁₉ (LaMgAl₁₁O₁₉). In this ideal composition LaMgAl₁₁O₁₉,the compound has practically no more possibility in varying itscomposition (see the phase diagram of FIG. 3). The homogeneity region isillustrated in the phase diagram starting from the specimen 1) andextending to either side. The ideal composition of the phase LaMgAl₁₁O₁₉is found at the point with the designation 1), while the composition ofthe phase LaAl₁₁O₁₈ (binary system without the addition of MgO) can beread off from the lower line between Al₂O₃ and La₂O₃ in the region ofabout 90 mol-% Al₂O₃. The points 1), 2) and 3) in the phase diagramindicate the specimens which were subjected to corrosion tests.

[0063] The thermal insulating material generally follows the formulaM₂O₃-xMeO-yAl₂O₃, whereby M is lanthanum or neodymium and thecoefficients x,y represent the preferred ranges of the composition with0.2≦x≦3.3 and 10.0≦y≦13. The components of the ideal compositionLaMgAl₁₁O₁₉ can be read from the phase diagram at the point 1) withabout 7.1 mol-% La₂O₃, about 14.3 mol-% MgO and about 78.6 mol-Al₂O₃.

[0064] Thus, a decrease in the lattice vacancies can be achieved in thethermal insulating material of the present invention by doping with MeO(e.g. MgO). This means that the material with the composition MMeAl₁₁O₁₉has absolutely no crystal defects in the structure, or formulateddifferently, all of the vacancies in the structure are occupied by Mgand an additional O atom. This complete occupation of all lattice sitesin the structure leads to the desired high stability in the temperaturerange above 1100° C.

Verification

[0065] The magnetoplumbite phases can be relatively easily substantiatedby means of XRD (X-ray powder diffractometry), since the JCPDS cards(26-0873, see Table 1) for the LaMgAl₁₁O₁₉ phase and (33-0699, see Table2) for the La₁₁O₁₈ phase are known and the characteristic interferencesor the reflection signals, respectively, can be determined.

[0066] The magnetoplumbite phases are very easy to verify with X-raytechniques on the basis of the JCPDS cards, since very many reflectionsoccur and they are very characteristic in their arrangement for thestructure (see FIG. 7). In contrast to this, ZrO₂ only shows a verysimple diffraction pattern.

[0067] However, since the two compositions hardly differ in theirchemical composition and both crystallise in the same spatial group(P6₃/mmc), a discrimination of the chemical composition only on thebasis of X-ray powder diffractometry is difficult. Therefore, a separatechemical analysis must be carried out. Two EDX measurements performed inREM are shown in FIGS. 5 and 6, which each show a corrosion test withCr₂O₃ and NiO.

[0068] One can clearly see that all of the respective elements can bedetermined on the basis of their characteristic energy spectrum. Thecomposition of the magnetoplumbite phase composed of Al, Mg, La and O isclearly recognisable. A quantitative evaluation of the EDX valuesresults in the composition of the magnetoplumbite phase. A typical X-raydiffraction measurement of the produced LaMgAl₁₁O₁₉ specimen is shown asan example in FIG. 7. A comparison with the JCPDS card (26-0873)according to Table 1 shows complete agreement.

Properties

[0069] The driving force for sintering, also present in these materials,then mainly leads to an enlargement of the grain size or graincoarseness in two dimensions (see FIG. 4). Because of this sinteringbehaviour, the cover layer does not become more dense as a whole.Rather, an enlargement of individual pores is more likely during thepost-sintering process. The porosity of the layer remains unchanged evenat temperatures of about 1400° C. (see FIGS. 12 to 15). The functionalproperty of being a heat barrier does not change even after the grainhas become more coarse.

[0070] Due to the plate-like structure of the material, cavities in themicrometer and submicrometer range are formed, which lead to a very lowheat conductivity of the layer (λ_(RT)=0.8-2.2 [W/mK], λ₁₂₀₀=1.2-2.6[W/mk] in the application temperature range. The thermal expansioncoefficient of the thermal insulating material lies between 9.5 and10.7×10⁻⁶ [K⁻¹] in the temperature range between room temperature and1200° C. and is thus in the same order of magnitude as that for chromiumnickel steels.

[0071] Also due to this structure, the E modulus of the layer increasesduring aging, but substantially slower in comparison to the conventionalzirconium oxide. To investigate this, one ZrO₂ specimen and oneLaMgAl₁₁O₁₉ specimen were left adjacent to one another in an oven at1650° C. to 1690° C. for 100 hours in the presence of air. The E modulusof La magnetoplumbite increased during the test by only half of that forzirconium oxide. This quite substantially leads to reducedthermomechanical stress, which in practice arises between the thermalinsulating layer and the metallic substrate, since due to the reducedstability crack structures would more likely form in the ceramic thermalinsulating layer, and thus a surface peeling of the ceramic layer causedby induced stress is effectively countered. The results of the E modulusmeasurements in comparison are compiled in Table 3.

[0072] A further important advantage of the thermal insulating materialof the present invention is that it is inert against the attack ofalkali compounds in the atmosphere (Na₂O, NaCl, K₂O, KCl).

[0073] Previous thermal insulating layers based on ZrO₂ form low meltingpoint phases with the hydroxides or carbonates of Na₂O and K₂O orthrough NaCl present in the atmosphere in winter or near the sea, whichlead to an enhanced densification of the thermal insulating layer attemperatures under 1000° C. However, with the material according to thepresent invention, such conditions more likely lead to an increasedplate growth, which subsequently makes the increase in density, i.e. thesintering of the cover layer substantially more difficult.

Powder Production a) Mixed Oxide Method

[0074] A first, particularly simple possibility of producing powder isthe use of the mixed oxide method, by which the corresponding oxides orsalts of the individual compounds as the starting materials are mixed ashomogeneously as possible in a drum, swing or tumbling grinder. Themixing process can be carried out in the wet or dry condition (see FIG.8). Aluminum oxide or zirconium oxide grains are preferably used in bothcases as the grinding bodies. Subsequently, the powder is granulated.When mixed in a liquid medium, preferably water, the resultingsuspension is then evaporated in a spray drier. Subsequently, the powderis preferably annealed in air at temperatures between 500° C. and 1800°C. for about 1 to 20 hours to form single-phase, oxidic agglomerateswith an average diameter of 1-200 μm and a specific surface area between0.1 and 40 m²/g.

[0075] The powder produced under dry conditions remains multi-phasedeven after the mixing process. This multiphase oxide mixture ispreferably initially treated with binders, before the granulation,preferably by spray drying, and the subsequent annealing treatment. Theannealing takes place in air above 500° C. up to about 1600° C. forabout 0.5 to 20 hours, whereby a homogeneous oxidic powder is formed. Inthis case, granulates are also formed with an average diameter ofbetween 1-200 μm and a specific surface area of between 0.1 and 40 m²/g.

b) Coating an Insoluble Carrier Powder

[0076] The powder can also be produced in an a wet chemical process fromoxides, hydroxides, acetates, carbonates, hydrogen carbonates or anothersalt as the starting compound (see FIG. 9). Initially, an insolublecarrier powder is coated.

[0077] One preferably works in an aqueous medium. Preferably aninsoluble oxide, a hydroxide or an oxy-hydrate of Al₂O₃ is used as thestarting material. The other components are added as water solublesalts, preferably carbonates, hydrogen carbonates or acetates. Followinga grinding and dispersion step, the resulting suspension is dried,preferably in a spray drying process and the powder is subsequentlysubjected to an annealing treatment. The powder is annealed attemperatures of 500° C. to 1800° C. in the presence of air for aduration of 1 to 20 hours to form single-phase, oxidic agglomerateshaving an average diameter of between 1-200 μm and a specific surfacearea of between 0.1 and 40 m²/g.

[0078] As an alternative medium, alcoholic solutions such as methanol,ethanol or isopropanol can also be used.

c) Production from Alcoholates (Sol-Gel Process)

[0079] In contrast to the above two variations, this route has theadvantage that the produced powder is extremely homogeneous in itscomposition and has very fine grains.

[0080] For the production, one expediently uses aluminum alcoholates andlanthanum or neodymium and Me alcoholates (Mg alcoholates), i.e.compounds which are already liquid or are soluble in alcohol and/orwater (see FIG. 10). By adding water to the alcohol solvent or bymodifying the pH value of an aqueous solution, the compounds areprecipitated and form very fine grain and very homogeneous mixturestogether. These are then separated from the solution and dried. Afterthe drying step, which can be carried out at temperatures in the rangeof about 500° C. to 1700° C., preferably in the range of about 1000° C.,very fine grained oxidic mixtures are formed.

[0081] According to a variation of the method, it is possible to adddispersing agents or binders after precipitation of the oxide to producea sprayable suspension as illustrated in FIG. 10. Subsequently, thegranulation of the powder follows, preferably in a spray drier, andfollowing this a calcination of the sprayed granulate at temperaturespreferably in the range of about 1000° C. to 1700° C. After separatingthe precipitated oxide or hydroxide from the remaining solution and theoptional addition of dispersing agents or binders, one obtains a masswith about 60 to 70% solids content, which can be readily spray dried.

[0082] For the alcoholates, one mainly uses compounds of the form—OC_(n)H_(2n+1), whereby the abbreviation OC_(n)H_(2n+1) stands formethoxy, ethoxy, isopropoxy, propoxy, butoxy and isobutoxy (with n=1 to5), or water soluble salts of lanthanum and magnesium (acetates,citrates, carbonates, hydrogen carbonates, formates, hydroxides,nitrates or other water or alcohol soluble salts) are added to asolution of aluminum alcoholate and are simultaneously precipitated,usually with the addition of water.

d) Production of Highly Crystallisable powder

[0083] During the mixing and drying as described above, the powder isadditionally doped with a different phase insoluble in the“hexa-aluminate” phase by adding a second component. This results in apreferred crystallisation of the aluminate during plasma spraying,increased adhesion and better thermal shock resistance.

[0084] These additives can include the following:

[0085] ZrO₂ as monoclinic, tetragonal or cubic form (the latter twophases each doped with MgO, CaO or Y₂O₃, respectively),

[0086] La₂Zr₂O₇, MgZrO₃, Nd₂O₃, HfO₂, Y₂O₃, Yb₂O₃, Eu₂O₃, La₂Hf₂O₇MgHfO₃,

[0087] salts of alkali oxides (Na₂O, K₂O, Li₂O).

[0088] The salts consist of carbonates, hydroxides, chlorides, nitrates,acetates, formates, citrates, sulphates, hydrogen carbonates or mixedsalts of the above-mentioned salts. Alloys or mixtures of thesesubstances are also possible. The amount of doping can lie between 0.001to 20 wt.-%, preferably 0.1 to 3 wt.-%.

[0089] The addition takes place either as a further oxidic powder in thewet chemical or in the mixed oxide method according to FIG. 11 or in theform of soluble components in the sol-gel method which are subsequentlyprecipitated during the powder production.

[0090] In the sol-gel route, the salt is also added and precipitatedduring the powder production or it is added with the binder anddispersing agents, and this mixture is then spray dried.

Processing of the Produced Powder

[0091] The preferred use of the thermal insulating material is thepreparation of thermal insulating layers on highly thermally loadedmetal parts, for example high alloy chromium nickel steels. The mainarea of application is for gas turbines in aeroplanes or turbines inthermoelectric power plants as well as thermally loaded components inmotors. Movable and non-movable parts can be coated. Higher degrees ofefficiency are achieved with these layers, because higher operationaltemperatures are possible. The wear on the machines caused by hightemperatures is greatly reduced.

[0092] Preferably, the coatings are applied by plasma spraying thepowder whereby the addition of the second component is preferred asmentioned. The resulting ability to crystallize during the rapid coolingby thermal spraying is improved, especially at the transition surface tothe metal layer. In addition, the adhesive properties are improved aswell as the thermal shock resistance.

[0093] Furthermore, high temperature resistance, thermal insulatingmassive parts can be produced from the powder, also with powdertechnology methods. The normal powder moulding methods are available,for example cold pressing or isostatic cold pressing, slip casting andthe like, after which the sintering process follows in a resistance ovenor a gas fired oven, optionally in the presence of a slightly reducingatmosphere, at temperatures in the range of above 1600° C.

[0094] Alternatively, ceramic foams can be produced. A polymer foam isfilled with slip and the solvent is driven off at temperatures of about200° C. to 300° C. This is followed by heating to about 1000° C. andthen a final heat treatment at about 1400° C. to 1700° C.

[0095] Alternatively, a suspension can be foamed into a low viscositypolymer with a foaming gas (for example polyurethane with foaminggas/hardening agent). Subsequently, the polymers are driven off attemperatures of about 1000° C. and finally the heat treatment is carriedout at about 1400° C. to 1700° C. TABLE 1 26-873 JCPDS-ICDD Copyright(c) 1996 PDF-2 Sets 1-46 database Quality: i d Å Int. h k l MgAl LaO 4.755 1 0 1 11 19 4.42 40 1 0 2 Magnesium Aluminum Lanthanum Oxide 4.03 201 0 3 3.66 20 0 0 6 3.63 15 1 0 4 Rad: Cu Lambda: 1.54056 Filter: Nid-sp: 3.25 15 1 0 5 Cutoff: Int: Diffractometer I/Icor: 2.79 70 1 1 0Ref: Verstegen, Philips, Bindhoven, Netherlands, Private Communication2.74 15 0 0 8 2.705 30 1 1 2 2.630 85 1 0 7 Sys: Hexagonal S.G.: P63/nmc(194) 2.485 100 1 1 4 a: 5.582 b: c: 21.942 A: C: 3.9308 2.420 30 2 0 0A: B: C: Z: 2 np: 2.295 50 2 0 3 Ref: Ibid. 2.210 10 2 0 4 Dx: 4.285 Dm:SS/POM: P30 = 16(.041,47) 2.190 20 0 0 10 ea: nvB: ey: Sign: 2V: 2.18015 1 0 9 Ref: 2.115 65 2 0 5 2.015 45 2 0 6 2.000 10 1 0 10 1.950 5 1 18 Color: White 1.842 10 1 0 11 Sample V 789-2 fired from the oxides at900 C. and 1550 C.. Magnetoplumbite 1.820 5 2 1 1 type. PSC: hP64. Nwt:764.00. Volume[CD]: 592.09. 1.773 5 2 1 3 1.732 10 2 1 4 1.725 10 1 1 10d Å Int. h k l d Å Int. h k l d Å Int. h k l 1.717 10 2 0 9 1.579 40 2 17 1.685 5 2 1 5 1.575 20 3 0 3 1.613 10 3 0 0 1.545 30 3 0 4 1.595 20 30 2 1.531 30 2 0 11 1.590 5 1 0 13

[0096] TABLE 2 33-699 JCPDS-ICDD Copyright (c) 1996 PDF-2 Sets 1-46database Quality: * d Å Int. h k l LaAl 0 11.02 16 0 0 2 11 18 5.51 6 00 4 Lanthanum Aluminum Oxide 4.81 4 0 1 0 4.71 30 0 1 1 4.41 30 0 1 2Rad: CuKa Lambda: 1.5418 Filter: Ni d-sp: Diff. 4.03 10 0 1 3 Cutoff:Int: Diffractometer I/Icor; 3.67 20 0 0 6 Ref: Ropp, R., Libowitz, J.Am. Ceram. Soc., 61 473 (1978) 3.63 11 0 1 4 3.25 5 0 1 5 2.781 45 1 1 0Sys: Hexagonal S.G.: P63/mac (194) 2.755 15 0 0 8 a: 5.561(7) b: c:22.041(4) A: C: 3.9635 2.696 15 1 1 2 A: B: C: Z: np: *2000 deg. 2.635100 0 1 7 Ret: Ibid. 2.482 75 1 1 4 Dx: Dm: SS/POM: P30 = 76(.007,54)2.408 6 0 2 0 ea: nvB: ey: Sign: 2V: 2.391 1 0 1 8 Ref: 2.352 1 0 2 22.288 25 0 2 3 Made by heating La2O3 and Al2O3 together at 1450 deg.for >130 days or at 2.113 65 0 2 5 1650 deg. for 96 hours. The data wasdeposited with J. Am. Ceram. Soc., 2.014 45 0 2 6 (ACSD146). KBr used asinternal standard. PSC: hP?. Plus 7 reflections to 1.850 8 0 1 11 0.96.Mvt: 723.69. Volume[CD]: 590.29. 1.814 4 1 2 1 1.728 5 1 2 4 d Å Int. hk l d Å Int. h k l d Å Int. h k l 1.717 15 0 2 9 1.3902 50 2 2 0 1.18676 0 1 18 1.605 5 0 3 0 1.3177 18 0 2 14 1.1615 5 0 4 5 1.576 35 1 2 71.2976 4 0 3 10 1.1433 4 1 2 15 1.5402 60 0 2 11 1.2406 6 1 2 13 1.14154 0 2 17 1.5187 4 1 2 8 1.2296 9 1 3 7

[0097] Comparison of the E modulus of ZrO₂ and LaMgAl₁₁O₁₉ after storageat 1670° C. for 100 hours in the presence of air TABLE 3 Material Emodulus tetragonal ZrO₂ specimen 242 GPa LaMgAl₁₁O₁₉ specimen 127 GPa

1. A thermal insulating material comprising at least one first componentwith at least one first phase, which stoichiometrically contains 1 to 80mol-% of M₂O₃, 0.5 to 80 mol-% MeO and a remainder of Al₂O₃ withincidental impurities, wherein M is selected from the group formed bylanthanum, neodymium and mixtures thereof, and wherein Me is selectedfrom the group formed by the alkaline earth metals, transition metals,the rare earths and mixtures thereof.